Through such an interrogating station, the transponders fitted on the objects are illuminated or charged by means of high-frequency interrogation signals. Such a transponder can be a passive transponder in which on the basis of the charge produced by the electromagnetic field, a transmission of its own identification code then takes place. The antenna used in the interrogating station in the state of the art has simultaneously both the function of a transmitting or interrogating antenna and the function of a receiving antenna for receiving the identification code. The antenna can be in the form of a frame antenna which is fitted round the conveyor, and which is generally satisfactory as regards to charging of the transponders, apart from a number of small dead zones for certain transponder antenna orientations. However, the accuracy and completeness of reading or receiving decreases if several transponders are placed in different configurations in close succession on the interrogation path. This is the case particularly in a noisy environment for which a shielding would then be necessary in receiving conditions.
An attempt can be made to reduce the dimensions of the frame antenna, as a result of which a more intense electromagnetic field is obtained, but the dimensions of the frame antenna are generally fixed by the dimensions of the conveyor belt or tunnel and the pieces of luggage being moved on or in it. For that reason such a frame antenna generally has to have dimensions on the order of 38.times.39 inches.